Electrical equipment or appliance connected to AC power grid line should satisfy current harmonic standard IEC61000-3-2. As shown in FIG. 1, With regard to different equipment or applications, IEC61000-3-2 has correspondingly set different current harmonic limits, among which, a Class A limit is for normal electrical equipment, Class B for portable tools and non-professional welding equipment, Class C for lighting equipment, and Class D for portable personal computer, monitor and TV.
Existing switching mode power supply technology, ranging from media power to high power, widely uses interleaved boost circuit to realize power factor correction in combination with full bridge circuit to realize DC-DC conversion.
As FIG. 1 shows, conventional AC-DC power supply comprises an AC-DC rectification circuit, a boost PFC converter and a DC-DC converter.
As FIG. 2 shows, interleaved boost circuit and full bridge DC-DC converter operate independently, and are controlled and driven by individual PWM controllers. In FIG. 2, first boost switching component Q1, inductor L1, diode D2, capacitors C1 and C2 construct a first boost circuit; second switching component Q2, inductor L2, diode D3, capacitors C1 and C2 construct a second boost circuit. The operating principal is as follows:
1) Q1 turns on: rectified input voltage on C2 applied on L1, energy stored in L1;
2) Q1 turns off: induced voltage on L1 and voltage on C2 superimpose to charge C1, voltage on C1 always higher than input instant voltage, so first boost circuit operates.
3) Q2 turns on: rectified input voltage on C2 applied on L2, energy stored in L2;
4) Q2 turns off: induced voltage on L2 and voltage on C2 superimpose to charge C1, so second boost circuit operates.
Q1 and Q2 conduct alternately. Such two interleaved boost circuits driven by Q1 and Q2, not only increase output power, but also reduce the ripple voltage at their output. The duty of Q1 and Q2, is controlled by PWM controller which senses and feedbacks PFC output (C1 voltage, normally design to 380V˜400V), and then generates 2 channels of PWM driving signal by conventional automatic control theory.
The above conventional boost circuit is capable of realizing power factor correction, to meet IEC61000-3-2 requirements, but it has the following demerits:
1. Need an individual PWM control circuit to improve power factor.
2. Need individual power supply circuits for PFC and PWM control circuits.
3. Need independent boost switching components (FETs and diodes) and PFC current sensing resistor R1.
4. Need PCB space, which is difficult in compact mechanical and layout design.
5. More components count, high cost.
6. Boost switching components Q1 and Q2 operate at hard switching mode, results in high loss, poor EMI noise.